2013 Tesla Model S: Making Sense of the Energy Displays
November 18, 2013
If you've been following the updates on our long-term 2013 Tesla Model S, you've likely seen a few pictures similar to the one above. This displayed squiggly line might look like it's charting Tesla's stock prices, but it's actually showing you real-time tracking of the car's energy consumption.
On one hand, it's easy to just drive the Tesla until you recharge (or run out of juice) and not worry about what your "fuel economy" is. But if you want to know what your Tesla is actually doing, and how your driving style affects it, you need to understand what's being displayed on this screen.
Given that the Model S is all-electric, you can forget all about miles per gallon. Instead, you'll be dealing with energy consumption and durations of distance and time.
The car's main energy tracking screen charts two things: watt-hours (Wh) on the vertical axis and distance in miles (mi) on the horizontal axis. Watt-hours is the way the Model S displays its energy consumption. But it helps to think of it with something familiar: If you have a 60-watt incandescent light bulb and you leave it on for one hour, it will use 60 watt-hours of energy. If you left a 1,000-watt light bulb on for one hour, that would be 1,000 watt-hours, or more commonly expressed in kilowatt-hours: 1 kWh.
In the picture above, you can see how the chart tracks energy consumption up to 900 Wh at the top. It will also show energy production (through the regenerative braking), expressed at a maximum of -300 Wh.
As you drive, the chart is continually updated. New tracking information is applied at the right edge of the screen (where mile "0" is), and that pushes older information off the screen at the left. Basically, peaks are when you're accelerating, and valleys are when you're decelerating/braking.
This information is then averaged. So on the left side of the screen, you'll see a figure: "Avg 278." This means the Model S, for the past 30 miles, has been averaging energy consumption of 278 Wh per mile. Again, thinking in familiar terms, this would mean that for each of those 30 miles the Model S has been driving, averaged out, it has been using the equivalent energy as if you had a 278-watt light bulb plugged in for one hour.
The Model S also knows how much energy it has left in its battery pack. Given that, and your average energy usage, it displays the "Projected Range" on the right side of the chart. In this case, if I kept driving exactly like I had been for the past 30 miles, I could go another 183 miles before I ran out of juice, according to the car.
This chart can be adjusted to different distance increments. You can also track average projected range or instant. The latter would be your range based on how you're driving the Model S at that very moment.
One interesting aspect is that the longer the distances you show on the chart, the more the graph gets smoothed out. To demonstrate, here's a picture of that same chart, but with the smallest, 5-mile increment.
This chart is only showing the past 5 miles driven. But you can see closer, more actual representations of energy usage and production. Right at the 3-mile mark, I slowed down to come to a stop at a stop light, and you can see some actual energy production here. Then, when the light turned green, I pressed down on the accelerator pedal (no gas pedal, right?) to swiftly accelerate up to about 60 mph. That was the big energy spike that went off the chart. Normally, I wouldn't have done this, but it was a good way to show how the chart reflects your real world driving.
Finally, all of this energy usage (and production) gets continually tallied and is then displayed on the main instrument cluster. You can see this information in the picture below.
Since my last charge, I've driven 50 miles exactly. And for those 50 miles, I've averaged 292 Wh per mile for energy consumption. If you multiply those numbers, you get 14,600 Wh, or 14.6 kWh, which is what the car is additionally displaying.
With either figure, you're very close to seeing how the EPA estimates "fuel economy" for electric cars. The standardized form is kWh used per 100 miles driven. The EPA combined city/highway estimate for the big battery pack (85-kWh) Model S is 38 kWh used per 100 miles. So, if you take my 292 Wh/m and multiply by 100 and convert into kilowatts, you'd get 29.2 kWh used per 100 miles.
Based on this it would seem that I'm "beating" the EPA estimate, or for those 50 miles driven, anyway. (Remember, the lower the energy consumption number, the better economy you're getting.) However, it's not a fair comparison. The EPA estimate includes charging losses, as this is a reality with an electric car, there's a certain level of unavoidable inefficiency. The in-car displays ignore this.
To get an apples-to-apples comparison, you would need to compare the in-car meter reading at the end for kWh used to the meter display on our HPWC or chargepoint, which will be a higher number. That higher number is the one that has to be recast into kWh/100 miles to truly compare to EPA ratings.
Still, the in-car information is quite useful for understanding where and when energy is being consumed and generated, and you can tailor your driving style to make the most of it.
Brent Romans, Senior Automotive Editor @ 11,741 miles